Add CBS constant for Drop Shipping

src/main/edu/wisc/services/cbs/common/Shipping.php

@@ -14,6 +14,9 @@ abstract class Shipping
     /** @var string CBS value for Campus Delivery shipping method */
     const CAMPUS_DELIVERY = 'CAMPUS_DELIVERY';
 
+    /** @var string CBS value for Drop Shipping */
+    const DROPSHIP = 'DROPSHIP';
+
     /** @var string CBS value for Tech Store Pickup shipping method */
     const TS_PICKUP = 'TECHSTORE PICKUP';